1. Field of the Invention
The present invention relates generally to the synthesis of high frequency signals. More particularly, the present invention relates to the synthesis of high frequency local oscillator signals for wireless communication applications.
2. Description of the Related Art
Wireless communication systems typically require frequency synthesis in both the received path circuitry and the transmit path circuitry. Phase locked loop (PLL) circuits including voltage control oscillators (VCO) are of use in mobile unit applications to produce a desired output frequency fout. As depicted in FIG. 1, a typical PLL circuitry 100 uses a phase/frequency detector 102 for detecting a frequency and/or phase difference between a reference frequency fref and the divided output frequency to drive a charge pump 104. This charge pump 104 delivers packets of charge proportional to the phase difference and/or frequency difference to a loop filter 106. The loop filter 106 outputs a voltage that is connected to the VCO 108 to control its output frequency fout. The action of this feedback loop attempts to drive the phase difference and/or frequency difference to 0 (or at least to a constant value) to provide a stable output frequency fout.
However, the performance of the communication system is critically dependent on the purity of the synthesized high frequency output signals. For signal reception, impure frequency sources result in mixing of undesired channels into the desired channel signal. For signal transmission, impure frequency sources create interference in neighboring channels. Therefore, a frequency synthesizer must typically meet very stringent requirements for spectral purity. The level of spectral purity required in cellular telephone applications make the design of a PLL synthesizer solution and in particular, the design of a VCO within a PLL synthesizer solution quite demanding.
Three types of spectral impurity will typically occur in VCO circuits that are used in PLL implementations for frequency synthesis: harmonic distortion terms associated with output frequency, spurious tones near the output frequency and phase noise centered on the output frequency.
Generally, harmonic distortion terms are not too troublesome because they occur far from the desired fundamental and their effects may be eliminated in cellular phone circuitry external to the frequency synthesizer. Spurious tones, however, often fall close to the fundamental. Phase noise is undesired energy spread continuously in the vicinity of the output frequency, invariably possessing a higher power density at frequencies closer to the fundamental of the output frequency. Phase noise is often the most damaging of the three to the spectral purity of the output frequency.
In PLL based frequency synthesis circuits, the steepness of the VCO frequency control characteristic determines the overall phase noise of the PLL. In order to provide a low VCO phase noise, the steepness should be as low as possible. However, as depicted in FIG. 2, a VCO frequency control characteristic 204 with a lower steepness can provide only a smaller frequency range 208 than a VCO frequency control characteristic 202 having a greater steepness. The minimal VCO steepness is determined by a summation of the frequency span of the VCO and the drift and the tolerance of the device parameters.